Further work will be performed on an in vitro system to demonstrate iron loss from mouse peritoneal macrophages after uptake of ferritin-Fe55 and ferritin-Fe55 rabbit anti-ferritin complexes, and homologous and heterologous erythrocytes labelled with hemoglobin-Fe55. Cellular events associated with iron storage and release will be explored with emphasis on lysosomal action and function, turnover of ferritin, and the role of transferrin at the cell surface. The mouse marrow macrophage after iron loading will be isolated. Light and ultrastructural morphology, light and ultrastructural histochemistry and functional studies will be performed on these cells.